"After investigating reports of compromised passwords, we have found that a small fraction of our user base has been affected," the dating website announced.

Sites like LinkedIn and eHarmony are attractive targets to criminals because the sites allow hackers to easily access personal data, the Washington Post reported. With LinkedIn, hackers can steal information from the site to use in email "phishing" scams.

Or if a hacker is malicious, then such a leak could give him the ability to engage in extortion, such as blackmailing people with details of their online love lives, Reuters reported.

"When somebody has the keys to your business and personal kingdom, that gives them all sorts of powerful information," Mary Landesman, senior researcher at security firm Cloudmark, told Reuters. "They might be able to use it for years."